Such a composite monolith, intended for example to form panels able to convey a fluid, antiballistic shielding structures, thermal protection devices, supports for optical devices for space applications, or refractory radomes for high-resolution radars, etc., is disclosed in document FR-A-2 749 327. In that document, the fibrous structure of the composite monolith is obtained by knitting, those parts of said fibrous structure that correspond to said skins being formed by rows of knitted stitches, whereas the part of said fibrous structure that corresponds to said spacers is formed from oblique knitting threads that connect stitches of one of the skins to stitches of the other of said skins.
Thus, this fibrous structure is in the form of two panels, formed from stitches, separated from each other and connected to each other by a plurality of oblique linking threads. It will be noted that this arrangement is particularly favorable for impregnation by the resin intended to form the matrix of the composite monolith since said linking threads between the two panels are free and directly accessible to the resin. However, in such a structure, the two panels may move relative to each other because of the flexibility of said linking threads, which raises difficulties as regards the positioning of said structure in the impregnation mold and in respect of the desired dimensions for the composite monolith.
In addition, it should be pointed out that this known composite monolith has limited mechanical properties since, in said skins, the high-strength fibers constituting the structure are looped and therefore can work neither in tension nor in compression, nor even in bending, which is why they are advantageously used in composite materials. Likewise, the oblique linking threads cannot provide this known composite monolith with the optimum crushing and swelling resistance.
Moreover, a double-walled composite monolith with filiform spacers is mentioned, incidentally, in document FR-A-2 718 670 which, principally, discloses a composite monolith obtained in the following manner:                a flexible fibrous reinforcement for each of said skins is applied to each of the opposed faces of a flexible core made of a material capable of being penetrated by a needle;        said fibrous reinforcements and said core are assembled by stitching by means of a thread forming oblique stitches that include thread portions passing transversely through said fibrous reinforcements and said core;        said sandwich structure is impregnated with the resin, which is intended to generate said matrix and which is then in the viscous state; and        the curing of said resin impregnating said fibrous reinforcements is carried out.        
This prior document also mentions that said core may, on the one hand, be made of a material impermeable to said resin, and, on the other hand, be removed after the resin curing operation, thereby allowing said composite monolith to be obtained.
However, there is no indication provided in document FR-A-2 718 670 as regards converting said transverse thread portions into filiform composite spacers. Now, the impregnation of said transverse thread portions cannot take place during impregnation of said fibrous reinforcements since, in this case, said transverse thread portions are isolated from the impregnation resin by said flexible core in which they are incorporated. To allow said transverse thread portions to be able to be impregnated with resin, in order to become composite rigid spacers, it is therefore necessary to assume that:                either a further impregnation with resin and a further curing of the latter are carried out after removal of said core;        or the stitching thread is preimpregnated with resin.        
In the first case, the additional impregnation and curing must take place while the transverse thread portions are still flexible, which, as mentioned above in regard to document FR-A-2 749 327, entails difficulties in positioning one skin relative to the other and defects in the monolith.
In the second case, the resin impregnating the preimpregnated stitching thread rapidly fouls the stitching machine, to the point of completely preventing it operating.
In addition, since the stitches in the structure of the above document are oblique relative to the fibrous reinforcements, such a structure does not have the optimum crushing and swelling resistance, just like the structure of document FR-A-2 749 327.